A mixer circuit with a volume control function is a typical component of audio integrated circuit chip design. An analog mixer circuit takes inputs from multiple analog audio sources, and mixes and sums those outputs. Attenuation of the analog audio source signals provides a means to control the volume output of the audio device.
A mixer circuit can use a tapped resistor string and an operational amplifier (op-amp) as shown in FIG. 1. Ideal op-amps have infinite input impedance and zero output impedance. Analog audio source signals enter as an input voltage (V.sub.in). A tapped resistor string is coupled through a switch to the inverting input (-) of an op-amp. The non-inverting input (+) is connected to a ground reference voltage (V.sub.cm). The circuit is a closed-loop inverting op-amp configuration in which some fraction of the output voltage (V.sub.o) is fed back to the inverting input terminal.
The characteristics of an op-amp and such closed-loop configurations are well known in the art. The output signal is driven to bring the inverting input (-) terminal to the same potential as the non-inverting input (+) terminal. The inverting input terminal is maintained at a virtual ground. In addition, the high input impedance minimizes the amount of current that can flow into the op-amp. The ratio between the feedback resistance and the input resistance determines the circuit's gain. The switches can be used to change the effective resistance and hence change the gain. They therefore act as a digital volume control.
A. When such a circuit is used for audio signals, the circuit's linearity specification is important, since poor linearity produces harmonic distortion in the audio output, which causes poor audio quality. Linearity specifies the deviation of the circuit's output-versus-input relationship from a perfect, straight line response.
However, there are trade-offs associated with the circuit configuration in FIG. 1 when a large range of attenuation is desired for a volume control. In order to provide greater than approximately 70 dB attenuation (excluding the single case of infinite attenuation), the system in FIG. 1 requires either a very large resistance (Ri) from Vin to the inverting (-) terminal or a very tiny feedback resistance (Rf) from the inverting (-) terminal to Vo.
However, such large resistance takes up a relatively large amount of chip real estate, and may affect other aspects of performance. In addition, an overly tiny resistance is difficult to implement practically in conjunction with the connections that must be made from the resistance to the switches and operational amplifier terminals. The present disclosure describes an alternative which allows obtaining an expanded attenuation range while avoiding both a large input resistance value and an overly tiny feedback resistance value.